1. Field of the Invention
The invention relates to millimetre wave imaging apparatus which may be used to identify objects concealed under clothing and, in particular, may be used in an indoor environment. The system may be of particular use as a security system for use in an airport.
2. Discussion of Prior Art
Existing techniques based on soft X-rays give good resolution but tend to be a health risk to those who are repeatedly exposed to the scanner, in particular the airport staff. A passive system is advantageous in that it avoids harmful emission. Furthermore, security systems typically used in airports, such as metal detector wands and archways, are unable to detect non-metallic objects such as explosives.
Millimetre waves have the property of passing through clothing and can therefore be used to create images of a person""s body and image items concealed beneath clothing. Passive millimetre wave imaging is a technique which uses emission and reflection from objects, and transmission through objects, of electromagnetic radiation in the millimetre wavelength region to construct an apparent temperature map or image of a scene or a person. The term passive imaging is usually used to describe imaging processes for which the levels of emission are comparable with natural background emission.
Using a millimetre wave imaging camera, images which have been recorded outdoors of a person concealing a gun beneath clothing clearly show the gun to be visible, since it reflects the cold sky temperature whereas the person""s apparent temperature is much closer to that of the surroundings. Other materials which have different properties from the skin of a person appear to have different temperatures from the person and so are visible in a millimetre wave image.
The image is built up of pixels which record the irradiance of electromagnetic radiation from a scene, which is focused onto one or more radiometers which may be scanned over a scene. The technique is particularly good at distinguishing metallic from non-metallic objects as metal is highly reflective at millimetre wavelengths whereas other materials tend to have much lower reflectivities.
Good millimetre wave imagery relies on suitable illumination of the subject. This is readily achieved outdoors, as the sky provides a suitable source of illumination, but in an indoor environment, such as an airport, this is not possible.
U.S. Pat. No. 5,227,800 describes a millimetre wave imaging system comprising an incoherent source of millimetre radiation, in the form of an array of Gunn diodes, each of which itself emits coherent radiation. The variations in manufacturing tolerances across the array of diodes provide the substantially incoherent radiation source. The system relies on illuminating panels consisting of many Gunn diodes to provide an illumination solid angle. However, in order to fully illuminate the subject from a full front hemisphere a large number of diodes are required. Additionally, as each diode is a point source, this leads to glints in the resultant image where radiation is specularly reflected from materials in the scene. Furthermore, as each diode is itself coherent, specular reflections may cause interference effects and hence speckle in the resultant image.
The present invention relates to millimetre wave imaging apparatus which overcomes these problems.
According to the present invention, apparatus for detecting at least one object having a millimetric temperature, situated in background surroundings having a background millimetric temperature, comprises;
means for varying the background millimetric temperature, so as to generate a contrast between the background millimetric temperature and the millimetric temperature of at least one object, and
detection means for detecting the contrast between the background millimetric temperature and the millimetric temperature of at least one object.
In one embodiment of the invention, the means for varying the background millimetric temperature may comprise;
at least one source, having an emitting surface, for emitting substantially incoherent millimetre wavelength radiation and
reflection means for reflecting millimetre wavelength radiation emitted by the source, such that the object is illuminated substantially uniformly with millimetre wavelength radiation.
The system may be used as a security system to identify items concealed under clothing, such as may be used in an airport or customs control point. The system provides an advantage over the prior art in that the emitting source can be made quite small and the resultant image does not suffer from glints where radiation is specularly reflected from materials in the scene. Furthermore, the bulk of the illumination solid angle may be provided by a cheap reflective material.
The reflection means may comprise an enclosure having a reflective inner surface. Preferably, the enclosure is a partial ellipsoid and the reflective inner surface may be embossed with a dimpled pattern, such that millimetre wavelength radiation reflected from the reflective inner surface is diffused.
In an alternative embodiment of the invention, the means for varying the background millimetric temperature may comprise at least one source having an emitting surface for emitting substantially incoherent millimetre wavelength radiation, wherein the emitting surface forms part of the enclosure, such that the object is illuminated substantially uniformly with millimetre wavelength radiation. For example, in this embodiment the enclosure may be a cubicle having at least one side formed from the emitting surface of the source.
The detection means in any embodiment of the invention may comprise at least one millimetre wave imaging camera for detecting millimetre wavelength radiation reflected from and emitted by at least one object. In one arrangement, the apparatus may comprise a single millimetre wave imaging camera having at least one millimetre wave centre frequency and bandwidth of operation. In an alternative arrangement, the apparatus may comprise two or more millimetre wave imaging cameras, wherein the imaging cameras have substantially different millimetre wave centre frequencies of operation. The one or more millimetre wave imaging camera may be mounted inside or outside the enclosure, although it may be preferable to mount it within the enclosure.
The one or more millimetre wave imaging camera may have an associated processor and television monitor for generating a millimetre wavelength image of at least one object.
In one embodiment of the invention, the source may comprise;
a layer of emissive material, having an optical thickness t, for emitting millimetre wavelength radiation and
heating means for increasing the temperature of the layer of emissive material,
whereby increasing the temperature of the layer of emissive material gives rise to an increase in the background millimetric temperature.
The heating means may comprise at least one heating element.
The source may also comprise a metal layer,
wherein the metal layer and the layer of emissive material form a structure having an emissivity, xcex5, in the millimetre wavelength region,
and wherein the optical thickness, t, of the layer of emissive material is such that the emission of millimetre wavelength radiation from said structure is substantially maximised.
The emissivity, xcex5, of the structure may be between 0.5 and 1 and, preferably may be substantially equal to 1. For example, the metal layer may be aluminium and the emissive material may be any one of clay tiles, asbestos fire blanket or fire cement.
The source may further comprise insulation means for reducing the emission of infrared radiation from the source. The insulation means may comprise a fibreglass mat. The source may also include at least one additional metal layer for internally reflecting millimetre wavelength radiation emitted from the emissive material, such that the emission of millimetre wavelength radiation from the emitting surface of the source may be substantially maximised.
In an alternative embodiment, the source may comprise;
a layer of material for emitting millimetre wavelength radiation and
cooling means for decreasing the temperature of the emissive material,
whereby decreasing the temperature of the layer of material gives rise to a decrease in the background millimetric temperature.
The source may also include a millimetre radiation transparent layer, such as a door, enclosing the emissive material. Typically, the layer of material may be radar absorbing material (RAM).
Alternatively, the source may comprise one or more Gunn diodes or a plurality of gas discharge noise sources for emitting substantially incoherent millimetre wavelength radiation. In addition, one or more optical elements may be included for diffusing the emitted millimetre wavelength radiation.
According to another aspect of the invention, a source for use in the millimetre wave imaging apparatus comprises;
a layer of emissive material, having an optical thickness t, for emitting millimetre wavelength radiation and
heating means for increasing the temperature of the layer of emissive material,
whereby increasing the temperature of the layer of emissive material gives rise to an increase in the background millimetric temperature.